Continuous high pressure tubular reactor polymerization plants convert relatively low cost olefin (generally ethylene) monomer feeds into valuable polyolefin product. The polymerization takes place at high temperatures and pressures and is highly exothermic. The high pressure polymerization takes place in the tubular reactor through which the polymerizing mixture passes in a turbulent, plug flow manner. The tubular reactor is constructed from a number of tube segments that are coupled together to make up the total tube length. As used herein the term “tube” refers to the totality of the tube segments that are coupled together.
The olefin monomer is generally ethylene but ethylenically copolymerizable monomers may be used as comonomers, such as vinyl acetate. The term “polymer” is used herein thus includes homopolymers and copolymers incorporating units derived from one or more comonomers. The term “polyethylene” refers to ethylene based polymers containing more than 50 mol % of ethylene derived units.
Typical commercial plants have total tube lengths varying from 500 to 3000 meters with a tube diameter of 20 to 100 mm and operate at 1500 to 3500 bar (150-350 MPa) and 120 to 350° C. Production is on a large scale, up to 200 kiloton (kT) per year and above. Electrically powered compressors may consume 25 megawatt (MW) or more annually of electricity. More energy is consumed in the form of steam to heat the feed to the “reaction start temperature” at which polymerization commences. This temperature is sometimes referred to as the “light off” temperature.
The plants generally use two compressors, each with multiple stages, arranged in series: a primary compressor provides an initial compression of the monomer feed and a secondary compressor increases the pressure generated by the primary compressor to the level at which polymerization takes place. The compressors may use intercoolers to shed the heat generated by compression.
The tube usually has an initial part, referred to herein as the “heating zone” to heat compressed monomer feed to the reaction start temperature. This is generally referred to as the “front feed.” A hot fluid, such as steam, flows through jackets surrounding the tube segments to effect heat exchange using the internal heat exchange surface area formed by the tube segments. Once the reaction start temperature is reached, initiator is injected into the tube and decomposes into free radicals that initiate polymerization. Parts of the tube downstream of the point at which the initiator is injected, form one or more reaction zones where polymerization takes place and the temperature rise caused by polymerization is counteracted by external cooling of the tube cooling or admixture of fresh monomer feed. The downstream portion of the tube is used to cool the compressed mixture of polymer and unreacted monomers for subsequent processing.
The terms “upstream” and “downstream” as used herein refer to the direction of the flow of monomer and polymer through the plant beginning with the monomer source, the compression and polymerization and terminating with the finishing stages for the polymer product, unless another meaning is clear from context.
A coolant fluid such as water may cool the tube externally. Heat exchange is effected using the internal heat exchange surface area formed by the tube segments that are cooled. The coolant fluid flows through cooling jackets surrounding the tube segments. The total internal surface area of the tube involved in the heat exchange is calculated (a) from the axial length of the tube subject to such cooling and (b) from the internal diameter of the bore, which may vary along the axial length tube to be considered.
In some types of plant the cooling fluid may be “cold” which means that the coolant is kept at a temperature below the melting point of the polymer. In other types of plant the cooling fluid may be “warm” or “hot” which means that the coolant is kept at a temperature above the melting point of the polymer. The cooling fluid is often pressurized water to allow cooling water to reach temperatures in excess of 100° C. The “waste” heat so removed by the coolant fluid can be used to generate heat or steam for other functions of the continuous polymerization process or for use outside of the polymerization process itself.
The efficiency of a continuous tubular reactor plant can be expressed in different ways. One measure is the amount of energy consumed per unit polymer produced, referred to as the “specific energy consumption”. Conventionally this is in the region of 0.8 kilowatt hours per kilogram of polyethylene (PE) produced for large scale reactors. The numerator depends primarily on the energy used to drive the compressors. Additional energy consumed in generating steam of sufficiently high pressure for heating feed to the reaction start temperature is not reflected in the specific energy consumption. The denominator is influenced by the extent of conversion of the feed into polymer along the length of the tube.
The “conversion” as used herein is the weight percentage of monomer fed into the tubular reactor that is converted into polymer during its passage through the tube based on the weight of monomer supplied to the tube. Another measure of plant efficiency is the nominal capacity that is the total amount of polymer that can be produced per year which is equivalent to the annual monomer consumption. The nominal capacity is a function of the compression capacity and the conversion.
Conversion may be increased by providing a number of reaction zones spaced along the length of the tubular reactor (see WO2007-018870 and WO2007-018871). Ethylene having a temperature less than that of the fluid inside the tube may be injected at such downstream reaction zones, thus cooling the fluid passing along the tube. These are generally referred to as “side feeds”. Conversion may be further increased by cooling these side feeds to increase the cooling effect (see, for example, paragraph [0003] in WO2007-018871) and reducing the reliance on external cooling through the tube walls as described above.
While the bulk of the reactor mixture passing through the tube, sometimes referred to as the “fluidum”, will be in single phase, the external cooling of the tube, especially with a cold coolant fluid, can result in fouling of the interior walls of the tube by polymer deposits. The foulant build up reduces the heat transfer and so the possible conversion of monomer to polymer in the tube. The build up of the foulant can be counteracted for example by maintaining a state of turbulent flow in the tube (see U.S. Pat. No. 3,628,918 and U.S. Pat. No. 4,175,169) and by pulsing (“bumping”) the flow through the tube (see U.S. Pat. No. 3,299,033 and U.S. Pat. No. 3,714,123).
Process control systems aim to provide the right mixture and temperature for each reaction zone using thermocouples at appropriate intervals in “rings” also referred to as “gaskets” clamped and sealed between adjacent tube segments. The temperature profile that results from the successive polymerization reactions and the cooling associated with the tube is measured by the thermocouples located in the rings. A target temperature profile can be maintained by varying the rate of injection, and types and combinations of free radical initiator, monomer and optionally transfer agent which is used to control the molecular weight of the polymer.
In conventional reactors the heated stream of tubular reactor effluent emerging at the downstream end of the tube is a single phase mixture containing polymer, unreacted monomer and residual transfer agent, if any. Conversions generally exceed 20% when using multiple reaction zones. The effluent first passes a pressure let down valve (referred to herein as the “let-down valve”) at the downstream end of the tube. The let-down valve lowers the effluent pressure so that the effluent is no longer in the single phase and starts to form two phases, a monomer rich phase with unreacted monomer and a polymer rich phase.
The two phase mixture is progressively separated in successive separation stages. Separated leftover reactants, such as unreacted monomer, are recycled to back to the compressors. The materials recycled can be referred to collectively as the “recycle gas”. The successive separation stages may include a cascade of separation vessels or “separators” which separate the polymer from the two-phase mixture until at the conclusion solid polymer pellets are obtained at ambient conditions of temperature and pressure. WO2007-018870 and WO2007-018871 show a conventional separation in a cascade of two vessels, a high pressure separator in the uppermost location (i.e. furthest upstream) and a low pressure separator in the lowermost location (i.e. furthest downstream), with the overhead of the high pressure separator conveyed to the inlet of a secondary compressor.
U.S. Pat. No. 4,342,853 describes the use of three separation stages, with an intermediate separator located between an high pressure separator and low pressure separator. In the specification and claims the intermediate separator is referred to as the medium pressure separator. The high pressure separator overhead is passed as a recycle gas stream to the secondary compressor. The overhead of the medium pressure separator is conveyed as a recycle gas stream to the inlet of the primary compressor with the purpose of improving energy efficiency and removal of residual gases. The manner in which the overhead from the low pressure separator is treated is not described.
GB1338280 mixes fresh ethylene after compression with the reacted mixture after pressure reduction to cool or “quench” the two phase effluent to avoid decomposition reactions, chain transfers, oxidation, branching and cross-linking that detract from the product quality, especially optical properties such as haze. U.S. Pat. No. 3,509,115 is similar.
WO2007-134671 uses an high pressure separator and low pressure separator only. Part of the kinetic energy of the reactor effluent is used to power a jet pump which takes part of the high pressure separator overhead recycle gas stream that has been cooled in a waste heat boiler to generate steam and recycle it to the high pressure separator. In this way, a cooled stream is provided that at least partially replaces the fresh ethylene stream of GB1338280 and U.S. Pat. No. 3,509,115. By using the jet pump to generate a quench stream from a cooled portion of the recycle gas, the need to compress monomer in the primary compressor to provide a quench stream is reduced.
WO2007-134670 discloses a separation and recycle arrangement using an intermediate separator, medium pressure separator between an high pressure separator and an low pressure separator. A jet pump is arranged so that the kinetic energy of the effluent is used to convey the medium pressure separator overhead and combine it with the effluent downstream of a let down valve. Unlike U.S. Pat. No. 4,342,853, which conveys the medium pressure separator overhead to the inlet of the primary compressor, in WO2007-134670 the medium pressure separator overhead is passed through the jet pump and high pressure separator to the recycle gas flowing to the inlet of the secondary compressor. In this way the need to recompress recycle gas in the primary compressor is reduced (see paragraph [0011]).
US2003-0114607 describes a continuous tubular reactor operation in which high conversions are pursued in combination with good optical properties such as haze, by injecting chain transfer agent preferentially with the front feed (see paragraph [0008] and [0010]). The dependence between the monomer concentration and chain transfer agent is uncoupled (see paragraph [0017]). In this way higher conversions can be combined with good optical properties.
It is among the objects of the invention to lower the specific energy consumption of a tubular reactor plant, and especially to lower the specific energy consumption without increasing tube length and nominal compressor duty. It is also among the objects to increase the amount of polymer produced for a given compressor capacity and/or to reduce the amount of electricity consumed by the compressors for producing the same amount of polymer. It is further among the objects to reduce the specific energy consumption and increase the conversion and plant capacity without reducing the polymer quality, especially the optical properties, such as haze. More specifically it is among the objects of the invention to improve increase conversion by permitting more reaction zones to be accommodated within a given tube length and improve extraction of unreacted components from the tube effluent.